Federal prosecutors have for months been using a grand jury to investigate former FBI deputy director Andrew McCabe — an indication that the probe into whether he misled officials exploring his role in a controversial media disclosure has intensified, two people familiar with the matter said.

The grand jury has summoned more than one witness, the people said, and the case is ongoing. The people declined to identify those who had been called to testify.

The presence of the grand jury shows that prosecutors are treating the matter seriously, locking in the accounts of witnesses who might later have to testify at a trial. But such panels are sometimes used only as investigative tools, and it remains unclear if McCabe will ultimately be charged.

A spokesman for the U.S. attorney’s office in D.C., which has been handling the probe, declined to comment, as did a spokeswoman for McCabe.

The investigation into McCabe is as politically charged as they come, and a decision to prosecute him — or not — will draw significant criticism either way.

McCabe — who briefly took command of the FBI after James B. Comey was fired last year — has been a frequent target of criticism from President Donald Trump. His comments, sometimes urging that McCabe be investigated, have bolstered McCabe’s argument that he is being treated unfairly, and that the examination of him is tainted by partisanship.

The special counsel’s office, though, has charged several former Trump campaign officials with allegedly misleading investigators examining Russian interference in the 2016 presidential election. If the Justice Department were to decline to take up the case against the FBI’s former No. 2 official, that could fuel outrage from conservatives who feel federal law enforcement has been unfairly aggressive toward their party. Using a grand jury could give federal prosecutors some political cover to argue that they pursued the case using the most forceful tools available to them and still came up empty-handed.

The allegations against McCabe come largely from Justice Department Inspector General Michael Horowitz, whose office concluded in a detailed report that McCabe had lied at least four times, three of them under oath, and that he had approved a media disclosure to advance his personal interests over those of the Justice Department.

Attorney General Jeff Sessions fired McCabe from the FBI in March — little more than 24 hours before McCabe was set to retire. McCabe derided the move, which cost him a significant portion of his retirement benefits, as one meant to slander him and undermine special counsel Robert Mueller’s investigation. McCabe launched an online campaign to raise money for his legal defense, collecting more than half a million dollars in less than a week.

At that time, the public knew only the broad outlines of what Horowitz had discovered. In mid-April, the inspector general made public his damaging report. Horowitz referred his findings to the U.S. attorney’s office in D.C., which at some point launched its own investigation into the former deputy director’s conduct.

McCabe’s lawyer said previously that the standard for a referral was “very low,” and he was “confident that, unless there is inappropriate pressure from high levels of the Administration, the U.S. attorney’s office will conclude that it should decline to prosecute.”

The inspector general’s investigation of McCabe focused largely on interactions that he authorized other FBI officials to have with a Wall Street Journal reporter in October 2016 and what McCabe would later tell investigators about those interactions.

The reporter — Devlin Barrett, who now works at The Washington Post — was preparing a story on tension inside the FBI and the Justice Department over two investigations related to Hillary Clinton. McCabe, apparently concerned the story would cast him as trying to shut down one of the probes, authorized the FBI’s top spokesman and an FBI lawyer, Lisa Page, to talk with Barrett for the story.

Page has since become more well known for the anti-Trump texts she exchanged with another FBI agent. In some of those texts, which the inspector general reviewed, she mentioned her conversation with Barrett.

Such interactions with the media, known as background conversations, are commonplace in Washington, and McCabe, as the FBI’s No. 2 official, had the authority to OK them. But the inspector general concluded that his doing so in October was meant to advance his own interests rather than those of the bureau, and that was inappropriate.

Perhaps more problematic, the inspector general also concluded that McCabe lied about having told Comey about his actions and to FBI and inspector general investigators who would later explore the matter. Lying to federal investigators is a crime that can carry a five-year prison sentence.

McCabe’s legal team has said previously that he did not intentionally deceive anyone — a point prosecutors would have to prove were they to bring a case — and his statements to investigators “are more properly understood as the result of misunderstanding, miscommunication and honest failures of recollection based on the swirl of events around him, statements which he subsequently corrected.”

They have also disputed Comey’s account of his interactions with McCabe, asserting Comey knew that McCabe was authorizing engagement with reporters. Prosecutors interviewed Comey in the case earlier this year.